1. Field of the Invention
The present invention relates to information acquiring devices, and in particular relates to an information acquiring device using a plurality of pulse lights with different center wavelengths.
2. Description of the Related Art
Various kinds of information can be obtained on a measurement object in a manner such that the object is irradiated with a pulse light and a light reflected or scattered by the object, a light passing through the object, or a light emitted from the object is detected for acquiring information.
In recent years, material identification has been an active area of research, which is conducted by irradiating a measurement object with pulses of light (pulse lights) with different center wavelengths and two variable different wavelengths, detecting lights generated by the object based on stimulated Raman scattering (SRS) or coherent anti-stokes Raman scattering (CARS), and obtaining a Raman spectrum. The stimulated Raman scattering and the coherent anti-stokes Raman scattering are hereinafter simply referred to as SRS and CARS, respectively, in some cases.
Optics Express, Vol. 20, No. 19, pp. 21010-21018, Sep. 10, 2012 (hereinafter referred to as Non-Patent Document 1) discloses a method of guiding a pulse light with a specific wavelength into a photonic crystal fiber having normal dispersion at the wavelength of the pulse light, thereby generating a wavelength-converted light with a narrow spectral bandwidth; and carrying out CARS imaging using the pulse lights with two different wavelengths corresponding to the wavelengths before and after the wavelength conversion.
Further, Japanese Patent Application Laid-Open No. 2011-180504 (hereinafter referred to as Patent Literature 1) discloses a method of guiding a pulse light with a specific wavelength into a plurality of photonic crystal fibers having anomalous dispersion at the wavelength of the pulse light, thereby generating a wavelength-converted light with a wide spectral bandwidth; and carrying out CARS imaging using the pulse lights with two different wavelengths corresponding to the wavelengths before and after the wavelength conversion.
However, there are shortcomings with the existing techniques described above. Specifically, the method disclosed in Non-Patent Literature 1 is disadvantageous in that since the wavelength of a light source is only variable in a small range, a CARS signal generated in a biological tissue has a small range of frequency.
Moreover, it is also disadvantageous in that since the output of the light source has a large noise component, the ratio of the noise to the CARS signal (signal-to-noise ratio) decreases.
In the meanwhile, the method disclosed in Patent Literature 1 is disadvantageous as well in that using photonic crystal fibers having anomalous dispersion (group velocity dispersion β2<0) causes that the spectral bandwidth after wavelength conversion is so wide that the utilization efficiency of the pulsed light used in CARS imaging is reduced.
Moreover, the method disclosed in Patent Literature 1 causes that the output of the light source has a large noise component and this reduces the signal-to-noise ratio.